1. Field of the Invention
The present invention generally relates to the field of semiconductor fabrication. More specifically, the invention relates to semiconductor wafer testing.
2. Related Art
In the field of semiconductor manufacturing, dies are formed on a semiconductor wafer and, following wafer fabrication, the dies are tested to identify the properly functioning ones prior to packaging. Packaging involves, among other things, dicing the wafer into separate dies and placing them in protective packages.
During wafer testing, every die is tested to identify those dies that meet requisite specifications, such as customer specifications, for example. During such wafer testing, a conventional tool, for example a probe card, is typically used to interface between the test equipment and a die under test. With a probe card, physical contact with the die under test is typically made by way of probe needles. Conventionally, the tips of the probe needles contact die bonding pads located on the surface of the die to establish electrical contact. Conventionally, probe needles are held in place by a dielectric material physically holding and separating the probe needles and also providing electrical isolation between probe needles. The base of each probe needle (the xe2x80x9cbasexe2x80x9d of each probe needle is situated at an end opposite to the tip of the probe needle) is connected to a corresponding contact on the probe card for interfacing with the test equipment. Conventional probe cards and probe needles, however, have a limited frequency range, which limits the probe cards"" ability to test a die. This limited frequency range is due, in part, to unwanted signal attenuation in, and crosstalk between, probe needles at frequencies over 500 mega Hertz (MHz). The unwanted signal attenuation and crosstalk are due to inductive and capacitive effects that are even more pronounced, and thus even less desirable, at higher frequency. As a consequence, conventional probe cards have limited usefulness for testing radio frequency (RF) devices which typically operate at high frequencies.
Accordingly, there is a need in the art for a probe card capable of adequately performing wafer tests at high frequencies. Furthermore, there is a need in the art for a probe card where signal attenuation in, and crosstalk between, probe needles are significantly reduced at high frequencies.
The present invention is directed to probe card and probe needle for high frequency testing. The invention overcomes the need in the art for a probe card capable of adequately performing wafer tests at high frequencies. Furthermore, the invention substantially reduces signal attenuation in, and crosstalk between, probe needles during high frequency testing.
According to one exemplary embodiment, a probe card comprises at least one shielded probe needle. The at least one shielded probe needle comprises a conductor, a dielectric sleeve surrounding at least a portion of the conductor, and a conductive cover surrounding the dielectric sleeve. During wafer testing, the tip of the conductor interfaces with a corresponding die bonding pad on a semiconductor die.
In accordance with the invention, the shielded probe needle operates as a coaxial transmission line and has a substantially constant characteristic impedance. This characteristic impedance is constant regardless of the signal frequency in the shielded probe needle. As such, signal attenuation at high frequencies is substantially reduced. In one embodiment, the characteristic impedance is approximately fifty ohms which is suitable for many applications. Also in the exemplary embodiment, the conductive cover is connected to a ground (or to a reference voltage) and shorts any current in the conductive cover to the ground (or to the reference voltage), thereby substantially reducing or eliminating crosstalk in the probe card.
An epoxy can be used to house and maintain the at least one shielded probe needle in a particular orientation so that the tip of the conductor makes precise contact with its corresponding die bonding pad on the semiconductor die during wafer testing. In one embodiment, the epoxy comprises a dielectric material. In other embodiments, the epoxy comprises conductive materials and is connected to the ground.